In medical and non-medical procedures, it often is desirable to ascertain the position of a plane of symmetry of an at least partially symmetrical body. In cranial and extra-cranial image-guided surgery (IGS), for example, it often is desirable to determine the mid-sagittal plane. Determining this plane is relatively easy, provided a three-dimensional image of the structure under observation is available. Such an image can be obtained, for example, via computer tomography or nuclear spin resonance recording. Methods for ascertaining the position of a plane of symmetry from three-dimensional images of structures are known in the prior art. Generally speaking, the plane of symmetry is found by determining which symmetrical structures of the three-dimensional body correspond to each other. Based on this determination, the plane of symmetry is calculated.
In hip surgery, determining the position of the mid-sagittal plane is very important, particularly if a new joint cavity is to be inserted into the hip. This is due to the fact that the exact position of the new joint cavity depends on various factors, one of which is the position of the mid-sagittal plane.
In determining the mid-sagittal plane of the hip, two characteristic points or so-called spinal points are established on the hip. Establishing these points, however, often is problematic since easy access to the characteristic points (for example by means of a pointer) is not possible due to the position of the patient (e.g., lying on their side) or due to tissue layers located over the hip and, therefore, over the characteristic points. Additionally, three dimensional images of the hip may not be readily available.